The present invention is directed to a hydraulically powered lift system for raising and lowering lift platforms of relatively large area.
In my U.S. Pat. No. 4,751,818, there is disclosed a hydraulic drive system operable to raise and lower a platen. The system there disclosed employs two or more lift cylinders having their piston rods coupled directly to the lift platen and a rotary hydraulic motor whose shaft drives a crank coupled by a link to the platen in a manner such that rotation of the crank from a six o'clock position relative to its axis to a twelve o'clock position will drive the platen from its lower end limit of movement to its upper end limit of movement. The lift cylinders and the rotary hydraulic motor are hydraulically connected in parallel with each other to a main system pump in a closed loop system via a three position directional control valve.
The geometry of the crank-link connection between the rotary motor shaft and the platen is such that the vertical component of movement transmitted by the link to the platen is relatively small when the crank is near its six o'clock or twelve o'clock position relative to the crank axis and is a maximum at the midpoint of rotation of the crank when the crank is at a nine o'clock orientation to the shaft axis. The effective vertical force applied via the crank varies inversely with its vertical component of movement. The lift cylinders, on the other hand, would normally drive at a constant speed and exert a constant lift force on the platen throughout their stroke. Because the piston rods of the lift cylinders and the output shaft of the rotary motor are both mechanically coupled to the lift platen, and the lift cylinders and rotary motor are hydraulically connected in parallel with each other to the main system pump, the output of the main system pump will be distributed between the rotary motor and lift cylinders so that the pressure applied to each remains constant--in other words, the system supplies proportionately more fluid to the cylinder or motor whichever is moving the fastest.
In a system of the type disclosed in my U.S. Pat. No. 4,751,818, where the platen is employed to advance or elevate a workpiece from a support into operative relationship with the tool, the relatively small size of the platen employed enables the lift system to employ a minimum number of cylinders and a single rotary motor. Further problems arise in the case where a lift platform of relatively large area is involved. Such platforms may be employed, for example, to lift single parts of relatively large length and width, as for example, a sheet metal automotive body panel or a workpiece lift and transfer system where the platform may be relatively narrow in width, but of substantial length due to the multiple number of work stations involved. In that these applications typically require a precise location of the lift platform relative to tooling which will operate upon workpieces supported by the platform, the elevating forces applied to the platform by the lift system must be uniformly and evenly distributed over the entire area of the platform insofar as is possible. Where the system employs an extensive linkage system, and a single motor is employed to drive a plurality of links connected to the platform at different points on the platform, those pivots in the linkage which are closer to the motor will tend to wear much faster than pivots which are more remote from the motor. This can result in an uneven distribution of force applied to the platform and, in the case of a relatively long platform result in a lengthwise tilting or sagging of the platform where uneven wear in the linkage occurs.
The present invention is directed to a hydraulically operated lift system especially adapted for lift platforms of relatively large area and which uniformly distributes the mechanical and hydraulic operating forces.